Polymer compositions for adhesive applications, like hot melt adhesive or pressure sensitive adhesive applications, are typically thermoplastic based adhesive compositions which are solid at room temperature, but melt quickly upon heating and then, upon cooling, set forming a firm bond e.g. on a substrate. E.g. a hot melt adhesive composition offers the possibility of almost instantaneous bonding which makes it highly suitable for automated production processes.
A hot melt adhesive composition includes typically a base polymer as the main component and other components like for example one or more of a tackifying resin, a plasticizer including wax, an additive(s) and/or a filler.
Important characteristics of a hot melt adhesive composition include, in addition to adhesion properties, softening temperature, cohesion properties and compatibility with other materials, just to mention few properties. Depending on the final application an appropriate hot melt adhesive composition is chosen so that it fulfils the requirements for that specific final application.
The base polymer in a hot melt adhesive composition is typically e.g. a polyolefin (ethylene- or propylene-based polymer), a functionalised polyolefin (ethylene or propylene copolymer with reactive groups), a styrene block copolymer, an ethylene vinyl acetate, etc. The base polymer the main component provides most of the features linked to the cohesive properties of the final HMA compound, e.g. strength, toughness, impact resistance, flexibility and mechanical properties at elevated temperatures.
The cohesion property is believed to be highly important for a hot melt adhesive composition in order to bring a durable solution to substrate movements and conditions appearing during the expected lifetime of the bonded component. The cohesion property means that the bond formed by the adhesive composition e.g. between two surfaces to be bonded together in an article has structural integrity and properties which enable the bond to distribute and withstand the stresses and strains and also enable energy dissipation in the end application use of the article. The man skilled in the art knows that the dissipation of energy within the adhesive composition plays one of the key roles in the cohesion properties of such adhesive composition. I.e. when pulling or shearing apart two surfaces bonded together by means of an adhesive composition of an article, the force which can be contributed to dissipation is normally considerably larger compared to the force due to the surface energy and/or a chemical bond between the substrate and the adhesive composition. Thus, better energy dissipation within the adhesive composition leads to better cohesion properties which then results in better overall bond performance (practical adhesion properties) of such adhesive composition.
Moreover, the cohesion properties of an adhesive composition are essentially provided by the base polymer as the main component.
Inter alia flexibility and elasticity are typically the key properties to determine the cohesive performance of an adhesive composition, like a hot melt adhesive composition. Both properties can be expressed in terms of tensile properties of the composition or polymer, like Tensile strength at yield point (MPa) and strain at yield point. The elasticity is normally defined as the tendency of a material or article to return to its original shape after being deformed. For elastic materials this is often linked to the E-modulus and to properties measured up to the yield point of the material, e.g. the E-modulus of elasticity determined for any point on stress-strain curve up to the yield point of the material as the ratio of tensile stress/tensile elongation, at this particular point, and at the yield point corresponding to the ratio of tensile stress at yield/tensile elongation at yield. The ratio is in fact corresponding to the Secant E-modulus when e.g. defined at a specific strain value.
Moreover, good flexibility and tailored elasticity level typically contribute to reduce the pulling force per unit area and to reduce the chances of the crack propagating within a hot melt adhesive composition. This means that said properties contribute in distributing and dissipating the pulling force thereby decreasing the failure of the overall adhesion capability in a hot melt adhesive composition.
Accordingly, the choice of the base polymer is vital in particular for the cohesion properties of the final adhesive composition. As the main component the base polymer typically provides the backbone to the adhesive composition and thus primary mechanical properties such as strength, like in tension and in shear, flexibility and elasticity and, normally, provides also the basis for the heat resistance to the adhesive, like hot melt adhesive, composition.
Additionally, it is important, but challenging, that an adhesive, like hot melt adhesive, composition provides both good cohesion and adhesive performance in order or meet the demands as a bond between two substrates in various end applications.
For example, if both the adhesive strength and the cohesive strength is high, the overall bond performance could despite this observation be poor; the bond needs also to show sufficient flexibility and energy dissipation (cohesive properties both) which are key ingredients for a good bond performance. Both the flexibility and the energy dissipation are important to enable movement and elimination of unnecessary high stresses in the bond between the substrates in the final composite part or product, due to stresses and strains applied to the composite part during its use in the application and during the entire lifetime of the product, e.g. via mechanical load or different thermal expansion coefficients of the various substrates.
The elastic recovery, i.e. the ability to recover in the same manner as an ideal rubber material is a most important feature for adhesive compounds. Particularly for structural bonding, construction, automotive and assembly etc., it is key that the bonded composite parts are strong as well as flexible and being able to recover back to its original shape after subject to high stress or substantial deformation and this particularly at elevated temperatures. The ability of the bond to recover to the original shape after stresses or strains are released is most often needed irrespective of how elastic or flexible to the bond is designed to be. For example, for comparatively rigid substrates like glass or steel or aluminium, the bond needs to be sufficiently elastic, flexible and allow energy dissipation during use of the composite product. However, if the bond is too elastic and too flexible, the structural integrity of the product could be jeopardized, meaning that the function of the product could be at risk if the dimensions are changed to a too large extent in the application. For comparatively flexible and low E-modulus substrates like e.g. textiles, soft polymers etc, the need for a bond with very flexible, soft and high elongation properties could be desirable. Practically, the type of substrates to be bonded and the application needs will determine the needs on the bond regarding flexibility, energy dissipation properties and ability to deform in elongation and/or in shear mode with low or high stresses. The ability of the bond to recover to its original shape after deformation is however almost always needed.
Accordingly, in addition to the cohesion properties, the base polymer contributes also to the adhesion properties based on the polymer structure and chemistry. However, conventionally in the state of the art, not only the base polymers, but also the other components, for instance tackifying resins, play a marked role for providing the adhesion properties for an adhesive, like hot melt adhesive, composition.
The hot melt adhesive compositions are used in a wide variety of applications, for example in combination with nonwoven materials such as for example disposable diapers and sanitary napkins, packaging such as for example case and carton sealing, bookbinding, bottle labelling, woodworking, textile and pressure sensitive application such as for example tapes, films and labels.
In the prior art for instance polyurethane (PUR) and moisture cured silane grafted amorphous polymer of alpha-olefin (APAO-R) has been used as the base polymer for adhesive compositions. The drawback of e.g. APAO-R is usually that only lower amounts, 0.5 wt % in maximum, of silane compound can usually be grafted thereto, which low silane content is not sufficient in many hot melt adhesive applications.
US20150240135 of Sika Technology discloses an adhesive composition suitable for vacuum laminating with uncoated aluminium tools [0030]. The composition comprises silane group-containing thermoplastic poly-alpha-olefin and at least one paraffin wax. The poly-alpha-olefin is defined in [0036] containing a priori no heteroatoms, such as oxygen, nitrogen or silicon, unless otherwise stated in the disclosure. Thus the silane compound, which can be e.g. vinyl trimethoxysilane, is introduced to the poly-alpha-olefin after the polymerisation thereof. The poly-alpha-olefin can be produced using Ziegler-Natta or metallocene catalyst and can be homopolymer of ethylene or homopolymer of propylene, see e.g. [0043-0046]. The introduction of the silane compound is carried out by grafting, see [0042]. Grafting is typically carried out by using a peroxide. The use of peroxide has drawbacks due the fact that e.g. simultaneously crosslinks the polyethylene causing an undesired increase in viscosity of the polyethylene component (worsens the processability and thus production rate of the composition). Moreover, the by-products of the peroxide may deteriorate the performance of the final end application, e.g. shorten the end use life of the article. The composition can additionally contain other poly-alpha-olefin polymer which does not contain silane groups, but which can contain e.g. (meth)acrylate comonomer or vinyl acetate, like EVA [0053]. The composition can further contain a catalyst that accelerates the reaction of silane groups (crosslinking), like organic phosphorous or tin compound [0057].
Accordingly, there is a continuous need to find new polymer compositions which would widen the product window in adhesive compositions and even improve the properties of the adhesive compositions during the production of articles and at the end use of thereof.